This invention relates to radar systems having a receive beam lobing capability and in particular to a method of discriminating between sidelobe and main beam reception of interference signal.
The traditional approach to sidelobe/main discrimination is to use a "guard horn", i.e., a second antenna which has an essentially omnidirectional receiving pattern. In this scheme, the guard horn omni pattern exceeds the sidelobe level of the "main" antenna, but has less power gain than the main beam. If the signal from the guard horn exceeds that from the main antenna, then the signal is being received by a sidelobe of the main antenna; otherwise it is being received by the main beam of the main antenna. Unfortunately, this approach makes use of two channels, one for the guard horn, and one for the main antenna, which along with the guard horn, makes the system more costly, heavier and larger than a single channel system.
Another approach, one which makes use of only one antenna, is possible when monopulse is used. In this approach a sum signal, an azimuth difference signal, and an elevation difference signal are produced for the purpose of target track via skin returns. In effect, separate sum and difference patterns are simultaneously produced. As a natural result of this approach, discrimination between interference received by the sidelobes and the main beam of sum pattern is made possible by comparison of sum and difference signal amplitudes. That is, the difference pattern effectively acts as a guard horn for the sum pattern, and the same logic applies as before. Unfortunately, this approach makes use of up to three channels; one for the sum, one for the azimuth difference, and one for the elevation difference. Again, therefore, there are shortcomings in cost, weight and volume in the overall design.
A third radar design concept, namely, single-channel, sequential LORO (lobe-on-receive only) neither has a guard horn nor monopulse signals. Instead, the receiving pattern is sequentially lobed "left", "right", "up" and "down" relative to the antenna main axis; in addition, the receiving pattern is aligned with the boresight in the search mode. Hence, five LORO positions are essentially available. Although it is effective and light in weight, this radar is also subject to the sidelobe/main beam discrimination problem.
One possible, but not totally satisfying approach to solving the problem, consists of desensitizing the receiver (i.e., automatically turning down its gain) during alternate (or otherwise sufficiently frequent) interpulse periods. This method determines that the interfering signal is being received in the sidelobe region of the search beam whenever the signal drops below a prescribed threshold (relative to system thermal noise) during the desensitized measurement. However, while this method may be able to distinguish between sidelobe and main beam reception of relatively strong interference, it cannot always correctly determine that a moderate or weak signal is received by the main beam alone, nor is the sidelobe/main beam breakpoint easily controlled by means of threshold comparison.
It is apparent from the foregoing that there currently exists the need for means to distinguish between sidelobe and main beam interference signals (including moderate and weak interference signals), which means must be implemented without adding appreciable cost and complexity to the radar system. The present invention is directed toward satisfying that need.